1. Field of the Invention
The present invention relates to an optical gyro for use in a detection of a rotational angular velocity. More particularly, it relates to a closed loop type fiber optical gyro using a serrodyne modulation and having a wide dynamic range of the detection, and an optical waveguide type phase modulator used in the fiber optical gyro.
2. Description of the Related Art
A typical fiber optical gyro using a serrodyne modulation includes a propagation path of an optical fiber through which light is propagated simultaneously clockwise and counterclockwise, an optical system optically coupled to the optical propagation path and having a serrodyne modulator for giving a serrodyne modulation to the propagated light, and a signal processing system for effecting an analog demodulation of a photoelectric output signal obtained from the optical system and thus driving the serrodyne modulator. In a known example, the signal processing system includes a unit for setting a frequency of a sawtooth waveform (or ramp waveform) signal for driving the serrodyne modulator, and a unit for generating the sawtooth waveform signal by means of an analog processing.
As described later in detail, however, the signal processing technique employing such an analog processing has a number of drawbacks. For example, since the sawtooth waveform signal is generated by means of the analog processing, a problem occurs in that a variable range of frequency of the sawtooth waveform signal is limited and thus a detectable range of the angular velocity of the gyro is limited to a narrow range. Another problem occurs in that an amplitude of the sawtooth waveform signal easily fluctuates due to a fluctuation in the temperature, or the like, and thus a linearity and scale factor stability of the gyro output are deteriorated.
Also, in a known optical waveguide type phase modulator used in an optical gyro, a problem occurs in that a bias fluctuation occurring due to light beams reflected from the end surface of the phase modulator is not negligible, as described later.
Note, the problems in the prior art will be explained later in detail in contrast with the preferred embodiments of the present invention.